To an increasing extent in recent years, tractors have become larger and larger. One tractor in common use today employs two sections articulated with respect to each other and an engine having over 250 drawbar horsepower. The reason for these very large tractors is that the farms on which they are used have greatly increased in size without any corresponding increase in the number of people doing the farming. It has accordingly become desirable to employ a very large tractor so that very wide implements can be pulled by it. This enables the ground to be covered in a shorter period of time by a single operator.
From a practical standpoint, there tends to be an upper limit on the size of the engine which may be used in a tractor. Consequently, as more and more power is necessary with ever larger tractors, some means must be developed for providing for a plurality of engines in connection with any one tractor. A number of arrangements have been developed in which there are one or more tractor vehicles serially connected behind one another, each tractor vehicle having its own engine. Also, it has been proposed to have two engine driven units, each driving their own ground engaging member, rigidly secured side by side. Both types of arrangements, however, have serious drawbacks. In the first place, if the vehicles are simply connected serially, this results in a train of vehicles which may become unduly long. Furthermore, all of the power has to be transmitted from the front vehicle through the train to the rear vehicle to which the implement is connected. Moreover, the implement must, of necessity, be connected through a rear vehicle and the more units there are in series, the further the implement will be from the operator. This has the drawback that if something is working improperly in the rear of the vehicle, it becomes more difficult for the operator who is in the lead tractor vehicle to see what is going wrong. In some cases, the implement requires an operator to ride with the implement. The farther back the implement is, the more difficult it becomes for the operator in the tractor and the operator in the implement to communicate with each other. Furthermore, where there are a number of vehicles connected serially, the wheels tend to follow in the same tracks and unduly pack the ground.
While, as indicated above, it is proposed to mount two units rigidly together side by side, this presents the problem that with very uneven ground, there is considerable twisting force exerted on the tractor.